Apparatus for producing annealed steels and process for producing said steels

ABSTRACT

An apparatus for producing annealed steels and annealed steels produced thereby.

This invention related to an apparatus for producing annealed steels andto a process for producing said steels.

Contemporary production processes at most steel manufactures arefocussed on high throughputs. High throughputs help to keep the costprice down, which is very important for commodity products like steel.However, the focus on low cost has an important drawback. High volumeproduction lines have inflexible processes and are unsuitable forproduction of high added-value niche products with process conditionsdeviating from the commodity products. The requirement for highthroughput imposes strict boundary conditions on the annealing cyclespossible. Because of this, new high strength steel (HSS) products needto be designed with strict limitations and are therefore always acompromise. It is difficult to run small size batches on these lines andin order to make a range of different products the chemistry needs to beadjusted to the process instead of the other way around. This hasresulted in a large variety of chemistries that are being used for thedifferent high strength steels currently produced and those underdevelopment.

Although alloy design is the most powerful tool available to productdevelopers the limitations imposed by customer specifications andin-house makability requirements (e.g. weldability, galvanisability,surface condition, mill loads etc) present a serious obstacle to furtherimprovement of existing products through alloying alone. Furthermore,these same limitations imposed on chemistry, when taken together withthe relatively restricted variation in annealing schedule which may beachieved over conventional high volume lines, represent hard obstaclesto commercialisation of the most promising metallurgical strategies forthe next generation of ultra high strength, high ductility steels. Inshort, current high strength steel developments are reaching theacceptable limits of alloy addition and the next generation of advancedhigh strength steel may not be achievable without resorting to alloycontents which are unacceptably high in the context of currentprocessing practice and capabilities.

Current HSS grades are often produced over conventional hot-dipgalvanising (HDG) lines with capacities of the order of several hundredthousand tonnes per annum. Advanced HSS (AHSS) strip is produced at suchcomparatively low volumes (up to several tens of thousands of tonnes perannum) that, in order to utilise such lines to their full capacity, itis necessary to accommodate a product mix comprising both AHSS andconventional HSS/low carbon steels. AHSS are multiphase steels whichcontain phases like martensite, bainite and retained austenite inquantities sufficient to produce unique mechanical properties. Comparedto conventional high strength steels, AHSS exhibit higher strengthvalues or a superior combination of high strength with good formability(Bleck & Phiu-on, HSLA Steels 2005, Sanya (China)). This inevitablyrequires that the designed annealing capabilities, of even those linesearmarked for HSS production, are a compromise across the wide rangingrequirements for production of a highly varied product mix. In order todeliver to specifications with sub-optimal and inflexible process alloydesigners are forced to do more with chemistry. From a metallurgicalstandpoint conventional HDG lines present several key technologicalbarriers to the production of truly optimised AHSS substrates which areboth inherent to the nature of high capacity lines and to the hot-dipgalvanising process itself:

-   1). Low cooling capacity/arrested cooling: Current lines employ    comparatively slow cooling and in all cases cooling is arrested at    an overage/zinc bath temperature.-   2). Fixed Overaging Duration: Current lines all incorporate a    cooling arrest either in the form of an extended overage or a zinc    bath dwell.-   3). Fixed Overaging Temperature: In conventional lines the overage    temperature is effectively imposed by the temperature of the Zinc    bath.-   4). Restricted Top Temperature: In conventional lines the maximum    top temperature may be limited by the installation and/or line speed    requirements.

Traditionally large volumes of relatively simple products were key to aneconomical operation of the large scale production facilities in themetal industry.

EP0688884-A1 discloses such a large scale production facility forannealing and hot dip galvanising a metal strip incorporating aninduction furnace which allows producing an initial temperature peak atthe beginning of the thermal cycle to accelerate the recrystallisationusing a heating zone consisting of an induction heating to the peaktemperature and a soaking zone (Z2) second with a cooling zone (Z1) inbetween.

With an increasing demand for the production of niche products at lowvolumes there is a need for more flexible production lines which areable to produce these low volume products economically. Currently suchflexible lines are not available.

It is an object of this invention to provide an apparatus for producingannealed steels which will allow the production of high strength steelswith simpler chemistries.

It is also an object of this invention to provide an apparatus forproducing annealed steels that allows to run small batch sizes againstrelatively low costs.

It is also an object of this invention to provide a process forproducing annealed steels using the said apparatus.

It is also an object of this invention to provide a process after themain annealing cycle that gives the option of applying additional localheat treatments.

One or more of the objects are reached with an apparatus for producingannealed steels comprising:

-   a. an uncoiler for uncoiling steel strip material-   b. a heating zone comprising:    -   i. a heating step comprising a first heating unit comprising of        radiant tube burners or an induction furnace for heating the        steel strip to a temperature of between 400 to 600° C. and a        second heating unit comprising one or more transversal induction        furnaces for further heating the steel strip to an annealing        temperature of between 500° C. up to about 1000° C.;    -   ii. a soaking step for soaking the steel strip for a period of        at most 120 seconds;    -   iii. a cooling step comprising a slow cooling zone, a fast        cooling zone and a third cooling zone, wherein the slow cooling        zone is for cooling the steel strip from the annealing        temperature to the fast cooling start temperature and wherein        the fast cooling zone is for quickly cooling the steel strip        from the fast cooling start temperature to a cooling stop        temperature of about 300° C. and wherein the third cooling zone        is for cooling the steel strip from the second cooling stop        temperature to a temperature of between ambient temperature and        100° C.;-   c. an optional reheating zone;-   d. an optional tailor annealing zone for local heat treating one or    more zones areas in the longitudinal direction of the strip;-   e. a final cooling zone;-   f. optionally a coating zone;-   g. a coiler for coiling the annealed strip material.

Preferable embodiments are provided in the dependent claims.

The apparatus according to the invention allows the development andproduction of (relatively) low-volume, high-value products instead oflow-value, high-volume products. The highly flexible continuousannealing and galvanising line is extremely useful because it allows theproduction of AHSS and UHSS steels with simpler chemistries and givesthe opportunity to run small batch sizes against relatively low(running) costs. The apparatus according to the invention allows theproduction of AHSS and UHSS steels with a flexibility of the heattreatments and thus in different properties over the length of thestrip.

A constraint of conventional production lines for the continuousprocessing of strip is that the heating and cooling is applied uniformlyover the whole width of the strip. One reason for this is to achieveuniformity in mechanical properties. However, it is often the case thatdifferent mechanical properties are required at different locations inthe product for its manufacture (e.g. formability as in bendability) orfor its application (e.g. high strength for energy absorption).Different mechanical properties can be achieved through differentheat-treatment cycles or post heat treatment after the main annealingcycle. Therefore, it would also be advantageous to incorporate not justflexibility in the temperature/time profiles of a production line, butalso allow the option of spatial flexibility in heat treating the stripwith multiple heat treatment zones parallel to the longitudinaldirection of the strip. The differences in heat treatment may bedifferences in overageing or tempering temperatures after a mainannealing cycle that may include a deep quench. The apparatus accordingto the invention allows the production of AHSS and UHSS steels with aspatial flexibility of the heat treatments and thus in differentproperties over the width of the strip. The latter local heat treatmentin a tailor annealing zone produces Tailor Annealed Strip (TAS).

The apparatus according to the invention provides the following newprocessing capabilities:

-   1). High top temperatures to enable full austenitisation;-   2). Rapid quenching to a range of temperatures including low (sub    Ms) temperatures;-   3). Re-heating to an overageing isotherm;-   4). Control over both overageing temperature and duration;-   5). Option of heat treating zones parallel to the strip length    having different temperature-time cycles or an additional post heat    treatment using a tailor annealing zone.

In particular UHSS substrates in many cases require full-austenitisation(high top temperatures) followed by rapid cooling to a low quenchtemperature and subsequent isothermal holding often at a temperaturesubstantially higher than the quench temperature.

For DP steels and other such partially martensitic grades a fastquenching capability is desirable for the formation of martensite. Thisreduces or eliminates the need of additions of alloying elements tosuppress unwanted transformations and ensure sufficient hardenability.Moreover, additions of hardenability elements such as C, Mn, Cr and Momay have significant implications for cost and for applicationsperformance, in particular weldability.

Within the family of HSS overageing requirements vary widely. For dualphase steels it is desirable to minimise the duration of overageing/zincbath dwell. In contrast, for TRIP or TRIP-assisted steels a controlledoverage is necessary to ensure the desired degree of austenitestabilisation and in turn the desired mechanical properties. Theapparatus accommodates these varying requirements.

In the case of both DP and TRIP Assisted steels optimisation ofsubstrate properties allows active control of the overageing temperatureand temperatures lower or higher than that of the zinc pot may beemployed.

The unique features of the apparatus are the capability to apply analmost endless variety of annealing curves and the possibility to switchquickly between production of different products. Both properties areenabled by the use of special technology that allows flexibility inheating and cooling sections of the furnace and a low heat latency ofthe furnace as a whole. The furnace is therefore the most important partof the line.

The heating zone of the line comprises a heating step, a soaking stepand a cooling step. This heating step comprises a first heating sectionthat will heat the product to an intermediate temperature. This firstheating section is followed by a second heating section that is able toheat the material to a temperature of around 1000° C. or a lowertemperature depending on the requirements. The intermediate temperatureis preferably between 400 and 600° C., and more preferably between 450and 550° C. A suitable intermediate temperature is about 500° C.

The first heating section preferably consists of a Radiant Tube Furnace(RTF). Alternatively an induction furnace could be used, but the RTFgenerally provide a more uniform temperature profile over the width atthese relatively low temperatures.

The second section preferably comprises one or more, but preferably atleast two induction heating sections in order to give the line itsheating flexibility. Most steel grades benefit from initial fast heatingin the temperature range between 500 and 750-800° C. Preferably this isenabled by a fast transverse flux (TFX) induction furnace following basetemperature heating up to 500° C. in the first heating part. The toptemperature between 850 and 1000° C. can be obtained by a second TFXinduction furnace. Because of the paramagnetic properties of some of thematerials (austenitic steels) transversal induction is needed. Thesecond TFX induction furnace is used for final heating from 800° C. toabout 1000° C. All ferrous materials become paramagnetic in thistemperature range, so transversal induction is needed. RTF cannot beused to heat to the top temperature because of the large thermal latencyin the cycle temperatures as a result of extensive heat accumulation inthe RTF equipment itself and the slower overall heating rate achievablewith RTF. This would adversely affect the flexibility of the apparatusin terms of rapid switches between annealing cycles.

The heating step is followed by a soaking step that is relevant for anumber of materials. It can soak materials at a given temperature forperiods depending on the line speed. The preferable maximum soaking timeis about 120 seconds, more preferably 60 seconds.

After soaking, the material will be cooled in the cooling step,preferably by three subsequent cooling sections: a slow cooling section,followed by a fast cooling section and finally a third cooling sectionthat will be active when materials need to be cooled to temperaturesaround 100° C. before entering the reheating zone.

Besides flexible heating also flexible cooling is needed to allow formaximum control in the creation of special microstructures containing amixture of austenite, ferrite and martensite. The cooling part, whichfollows after the soaking part, comprises one or more cooling sectionsto achieve the cooling of the strip after soaking. In an embodiment thiscooling part comprises a slow cooling section, a fast cooling sectionand a third cooling section. The slow cooling section is used to coolthe strip from the soaking temperature to the fast cooling starttemperature, which is usually just above the temperature where theaustenite would start to transform (Ar3). In the fast cooling section,the strip is cooled from the temperature just above Ar3 to a temperatureof about 300° C. The third cooling section would further cool the stripto a temperature below the temperature where no further transformationtakes place, i.e. about 100° C. The fast and third cooling section maybe separate sections, or one integrated section with the ability ofcontrolling the cooling stop temperature and the cooling rate. Thecooling rate in the fast cooling is preferably at least 50° C./s.

In the reheating zone the strip may be subjected to an overageing stepor an annealing step. In order to reach the overageing temperature in afast and flexible manner, another induction furnace is installed. Thereheating zone of the furnace can be used as an overageing section oroptionally, it can be used to apply a uniform or local heat treatment.The latter local heat treatment produces Tailor Annealed Strip (TAS). InTAS material, mechanical properties can be tailored according to thespecific requirements of the part. At locations where more formabilityis needed this can be achieved by local heat treatment of the strip inthe line, usually resulting in desired variations of the mechanicalproperties over the width of the strip. The products this TAS-optionwill enable are coils of strip of coated or uncoated HSS with one ormore zones parallel to the rolling direction. These zones are preferablyat least 50 mm wide. The properties of the TAS-treated zones will bedependent on the applied temperature cycle but will in general result inan enhanced (local) formability which can facilitate the use of HSS/UHSSfor complex part geometries. After the overaging, the uniform annealing,or the TAS treatment, the strip will be cooled to about between 150 and250° C. in a fourth cooling section before leaving the protectiveatmosphere. Finally the strip will be cooled with air to about 50 to100° C. in a fifth cooling section. Preferably the fourth coolingsection cools the strip to about between 150 and 250° C., preferablyabout 200° C., preferably using HNx and/or the fifth cooling sectioncools the strip to about 50 to 100° C., preferably about 80° C.,preferably by using air cooling.

The reheating to an overageing temperature of preferably between 350 and450° C. preferably takes place by means of a longitudinal flux induction(LFX) because of the flexibility it provides. As the relevant steels areall magnetic at the overageing temperatures there is no need to use aTFX-furnace, although it could be used instead of an LFX. For the tailorannealing zone a TFX-unit is needed as the temperatures involved ofpreferably between 750 and 850° C. involve paramagnetic materials. Theoverageing time depends on the line speed and the length of the furnace,but it is generally preferably limited to 180s.

The galvanisation is performed by electrolytic coating in anelectrolytic coating part. Electro-galvanising was chosen instead of hotdip galvanising. This was done in order to be able to make the annealingprocess completely independent of the galvanising process and to be ableto achieve an excellent coating quality even at lines speeds which arelow in comparison to conventional HDG lines. An activation/picklingand/or cleaning section is preferably used just before the anelectrolytic coating part. This reduces surface related problems to aminimum and allows the use of a larger variety of alloying elements.

It is preferable that annealing and coating steps are separated suchthat coating requirements (such as line speed and strip temperature) canbe met without consequence for the development of the substratemicrostructure or imposition of severe alloying restrictions. Besidethese advantages there is the obvious advantage that current highcapacity lines to produce large volumes of consistent commodities arerelieved of the production of these difficult niche-products.

According to a second aspect, the invention is also embodied in aprocess using the apparatus according to the invention.

According to a third aspect, the invention is also embodied in theannealed steel produced using the apparatus or the process according tothe invention.

By means of a non-limiting example, a schematic drawing of an apparatusin accordance with the invention is presented in FIG. 1.

In FIG. 1 the reference numbers refer to the following:

-   -   1. strip material    -   2. heating zone    -   3. entry zone    -   4. radiant tube furnace section for the heating step    -   5. TFX-section for the soaking step    -   6. cooling section for the cooling step    -   7. LFX reheating zone    -   8. overageing or TAS-zone    -   9. final cooling zone    -   10. coating zone    -   11. exit zone    -   12. uncoiler    -   13. coiler

The entry zone may e.g. comprise one or more of rinsing equipment,drying equipment, buffer means (such as looping tower). The exit zonemay e.g. comprise one or more of surface inspection, oiling equipment,cutting equipment or buffer means.

By means of non-limiting examples the flexibility of the apparatusaccording to FIG. 1 is demonstrated by means of FIGS. 2 to 6 wherein inFIG. 2 the thermal curve for a 600 MPa AHSS is presented comprisingferrite, bainite, martensite and retained austenite. FIG. 3 shows thecurve for a recovery annealed steel, FIG. 4 for a steel comprisingbainitic ferrite and martensite, and FIG. 5 for a tempered martensite.

FIG. 2: A fast heating rate in the temperature range 500-750° C. isemployed because fast heating through into the heating transformationrange is beneficial since it influences the size and distribution of theintercritical austenite and thus, in turn, of the second phase in thefinal microstructure. After the RTF furnace, the material is heated to˜750° C. Subsequently the strip goes through the 2nd fast heating to thesoaking section at an intercritical temperature typically in the range780-850° C. for. After soaking for ˜30 seconds, the strip is firstslowly cooled and then fast cooled to an overageing temperature of ˜420°C. This temperature is chosen to promote the formation of bainiteleading to the enrichment of carbon in austenite and thus the retentionof metastable austenite in the final microstructure. Martensite isformed in the final cool followed by cooling to ambient temperature. Aninterruption of the final quench at 200° C. or lower is permissible.

FIG. 3: Heat treatment of 10-60 s at 600-700° C. where the heating andcooling rates are not critical to induce recovery in a cold-rolled highstrength steel to allow for an increased elongation at the expense ofsome of the work-hardening.

FIG. 4: After the RTF furnace, the material is heated to ˜750° C., andafter the 2nd fast heating the strip will have a temperature>Ac3. Afterfull austenitizing during the soak at ˜850° C. for ˜30 seconds, thestrip is slowly cooled but the temperature should remain above 700° C.at the end of the slow cool section. The fast cooling will decrease thestrip temperature to <400° C. In the overageing section the austenitedecomposes virtually completely to bainitic ferrite such that nomartensite will be formed in the final cooling.

FIG. 5: First the material must be fully austenitic at temperaturedependent on the C and Mn content, but typically above 820° C., followedby relatively fast cooling of at least 80° C./s to below a temperatureof at least 200° C. to fully transform into martensite. Light temperingto improve bendability and hole-expansion can be achieved by re-heatingup to about 400-500° C. for 10-60s. Higher temperature or longertempering to improve formability at some expense to strength is achievedby heat treating at 600-750° C. for 30-60s. Heating and cooling ratesfor tempering are not critical.

FIG. 6: The strip is heated and austenitized in the intercritical regionmeaning that the soaking temperature is in the range 830-860° C. Thevolume fraction of intercritical ferrite is controlled by this toptemperature, which in its turn determines the hardenability of theaustenite prior to cooling. After the soak, the strip is cooled slowlyto ˜700° C., and subsequently the strip goes through the fast coolingsection to arrive at temperature near Ms (˜350° C.). For this productthe 3rd cooling section is important to cool the strip to ˜250° C. Amoderate cooling rate is sufficient in this section because theformation of martensite in this temperature range is not time dependentbut simply controlled by the undercooling below Ms. After cooling thestrip is heated by means of induction to enter the overageing section ata temperature of 350-450° C. During the isotherm for ˜70 seconds (1) theas-formed martensite is tempered, (2) the austenite may become morestable due to carbon partitioning and (3) some carbide-free bainite maybe formed which may also stabilise the austenite. For this product it isaimed to create very stable austenite, which means that no martensitewill be formed in the final cooling.

1. An apparatus for producing annealed steels comprising: a. an uncoilerfor uncoiling steel strip material b. a heating zone comprising: i. aheating section comprising: A. a first heating unit comprising ofradiant tube burners or an induction furnace for heating the steel stripto a temperature of between 400 to 600° C., and B. a second heating unitcomprising one or more transversal induction furnaces for furtherheating the steel strip to an annealing temperature of between 500° C.to about 1000° C.; ii. a soaking step for soaking the steel strip for aperiod of at most 120 seconds; iii. a cooling step comprising a slowcooling zone, a fast cooling zone, and a third cooling zone, wherein theslow cooling zone is for cooling the steel strip from the annealingtemperature to the fast cooling start temperature and wherein the fastcooling zone is for quickly cooling the steel strip from the fastcooling start temperature to a cooling stop temperature of about 300° C.and wherein the third cooling zone is for cooling the steel strip fromthe second cooling stop temperature to a temperature of between ambienttemperature and 100° C.; c. an optional reheating zone d. an optionaltailor annealing zone for local heat treating one or more zones areas inthe longitudinal direction of the strip e. a final cooling zone f. acoating zone comprising: optionally a pickling and/or activation stepoptionally a first cleaning step an electrolytic coating step optionallya second cleaning step optionally a drying step g. a coiler for coilingthe annealed strip material.
 2. The apparatus according to claim 1,wherein the heating zone comprises a first heating unit comprisingradiant tube burners for heating the steel strip to a temperature of atmost about 500° C.
 3. The apparatus according to claim 2, wherein thesecond heating unit comprises a first transversal induction furnace forfurther heating the steel strip to a temperature of up to about 800° C.and a second transversal indication furnace for further heating thesteel strip to an annealing temperature of at most about 1000° C.
 4. Theapparatus according to claim 1, wherein the cooling rate in the fastcooling zone is at least 50° C./s.
 5. The apparatus according to claim1, wherein the reheating zone comprises a longitudinal induction furnacefor reheating the steel strip to a temperature of between 350 and 550.6. The apparatus according to claim 1, wherein the reheating zonecomprises a partial heating zone comprises a transversal inductionfurnace for uniformly or locally reheating the steel strip to atemperature of between 700 to 900° C.
 7. The apparatus according toclaim 1, wherein the apparatus comprises a tailor annealing zone forlocal heat treating one or more areas in the longitudinal direction ofthe strip.
 8. The apparatus according to claim 1, wherein galvanisationof the steel is performed in the electrolytic coating step.
 9. A processfor producing an advanced high strength steel using the device accordingto claim 1 comprising: a. uncoiling steel strip material using anuncoiler b. heating the steel strip in a heating zone comprising: i. aheating section comprising: A. heating the steel strip to a temperatureof between 400 to 600° C. in a first heating unit comprising radianttube burners or an induction furnace, and B. further heating the steelstrip to an annealing temperature of between 500° C. to about 1000° C.in a second heating unit comprising one or more transversal inductionfurnaces; ii. a soaking step for soaking the steel strip for a period ofat most 120 seconds; iii. a cooling step comprising a slow cooling zone,a fast cooling zone, and a third cooling zone, wherein the slow coolingzone cools the steel strip from the annealing temperature to the fastcooling start temperature and wherein the fast cooling zone quicklycools the steel strip from the fast cooling start temperature to acooling stop temperature of about 300° C., and wherein the third coolingzone cools the steel strip from the second cooling stop temperature to atemperature of between ambient temperature and 100° C.; c. optionallyreheating in an optional reheating zone d. optional tailor annealing inan optional tailor annealing zone for local heat treating one or morezones areas in the longitudinal direction of the strip e. cooling in afinal cooling zone f. coating in a coating zone comprising: optionally apickling and/or activation step optionally a first cleaning step anelectrolytic coating step optionally a second cleaning step optionally adrying step g. coiling the annealed strip material using a coiler forcoiling the annealed strip material.
 10. The process of claim 9,comprising producing a tailor annealed advanced high strength steel withvarying properties over the width of the strip as a result of local heattreating one or more areas or zones in the longitudinal direction of thestrip using the tailor annealing zone for local heat treating one ormore areas in the longitudinal direction of the strip.
 11. The processaccording to claim 10, wherein the zones with the desired variations ofthe mechanical properties over the width of the strip and parallel tothe rolling direction are at least 50 mm wide.
 12. The process accordingto claim 9, wherein the strip is electrolytically galvanised in-line.13. The process according to claim 9, wherein the strip is heated to anintermediate temperature of between 400 and 600° C. in the first heatingunit and heated to a temperature of at most 1000° C. in second heatingunit, soaked for at most 120 s and cooled in the cooling section in aslow cooling section from the soaking temperature to the fast coolingstart temperature of just above Ar3 followed by cooling from thetemperature just above Ar3 to a temperature of about 300° C. in the fastcooling section at a cooling rate of at least 50° C./s followed bycooling the strip in the third cooling section to a temperature belowthe temperature where no further transformation takes place.
 14. Theprocess according to claim 9, wherein the strip is a cold-rolled steelstrip heated to intermediate temperature of between 400 and 600° C. inthe first heating unit and heated to temperature of between 600 and 700°C., heat treated for 10 to 60 s followed by cooling to produce arecovery annealed cold-rolled high strength steel.
 15. Annealed steelproduced using the apparatus of claim
 1. 16. Annealed steel producedusing the process of claim
 9. 17. The apparatus according to claim 1,wherein the reheating zone comprises a longitudinal induction furnacefor reheating the steel strip to a temperature of between 400 and 500°C.
 18. The apparatus according to claim 1, wherein the reheating zonecomprises a partial heating zone comprises a transversal inductionfurnace for uniformly or locally reheating the steel strip to atemperature of between 750 to 850° C.
 19. The apparatus according toclaim 1, wherein the apparatus comprises a tailor annealing zone islocated behind the heating zone.
 20. The process of claim 9, comprisingproducing a tailor annealed advanced high strength steel with varyingproperties over the width of the strip as a result of local heattreating one or more areas or zones in the longitudinal direction of thestrip using galvanisation of the steel performed in the electrolyticcoating step.
 21. Annealed steel produced according to the process ofclaim 9.